Title: Effects of climate, nutrients and vegetation on the trophic cascades in lakes; a study along a latitudinal gradient in South-America

Primary Researcher: Gissell Lacerot
Email address: gige@fcien.edu.uy
Submitted on: June 28, 2004

Start date: 01 July 2004
End date: 01 July 2008

Introduction:

Freshwater of good quality is an increasingly limiting resource in the world and South America is not an exception. Just like in temperate lakes, the majority of the water quality problems are related with an excessive nutrient loading. This does not only stimulate planktonic algae (phytoplankton) growth, but also produces a cascade of ecological effects resulting in strongly impoverished biodiversity of aquatic plants, invertebrates, fish and birds (Scheffer, 1998), not to mention the deterioration of the quality of water for human use. The proposed study focuses on shallow lakes, which are the dominant lake type in most flat areas of the world where they are closely linked to human societies. They are abundant in riverine and coastal zones where most of the population is concentrated, and intensively used for water supply, fish farming and recreation activities. They are also richly diverse habitats, with enormous importance for nature conservation.

The trophic cascade in temperate lakes

In addition to nutrient control, the indirect effect of fish on algae known as ‘the trophic cascade’ has become one of the key-mechanisms to control excessive development of phytoplankton in temperate shallow lakes. Zooplankton filters the water, controlling phytoplankton biomass. Especially large bodied species such as those from the genus Daphnia (“water fleas”) are tremendously effective in exerting this top-down control and their presence in a sufficient number can significantly increase water transparency. However, when fish predation is strong, these large-bodied zooplankters are the first ones to disappear and the zooplankton community structure shifts towards smaller-bodied species that are much less effective at controlling phytoplankton biomass (Brooks & Dodson, 1965). An application of this insight in temperate lakes is to reduce fish density in order to promote an increase in the abundance of water fleas, which then filter the water clear of phytoplankton, so-called “biomanipulation” (e.g. Lammens et al., 1990).

Excessive loading with nutrients (eutrophication) does not only stimulate algal growth directly, but has also an indirect effect as it affects the trophic cascade (Jeppesen et al., 1996a). In shallow eutrophic lakes the high productivity leads to an increase in the flow of organic material to the sediments. This organic matter represents a food source for a wide variety of small animals including fish, which uses it as an alternative choice. This boosts fish biomass, which indirectly hammers back on the Daphnia, as all new-born fish focus on these water fleas as food. Thus top-down control of algae by zooplankton is usually weak in such lakes, and as a result nothing stops the abundant algal growth stimulated by the high nutrient levels. A particularly important factor affecting the trophic cascade in temperate shallow lakes is the presence of submerged vegetation (Timms and Moss, 1984; Scheffer, 1998). Although the situation may vary depending on the vegetation density and the fish-species involved, aquatic macrophytes act as a refuge for large zooplankton, thus reducing the cascading effect of planktivorous fish.

Why the trophic cascade may work differently in warm climates

There are several indications that the trophic cascade theory developed for temperate lakes cannot be simply applied to (sub)tropical lakes. Recently, attention has been focussed on the observation that zooplankton species in tropical lakes and ponds are generally much smaller than in temperate zones and in particular large herbivorous zooplankton of the genus Daphnia seem less diverse or even absent at lower latitudes (Dumont, 1994; Gillooly & Dodson, 2000). As a consequence, chances for an effective top-down control over phytoplankton are likely to be diminished at lower latitudes (Crisman & Beaver, 1990; Fernando, 1994; Lazzaro, 1997; Pinel-Alloul et al. 1998). Several possible explanations have been suggested for the observed pattern. It could be related to a lower-upper thermal tolerance of these organisms (Moore et al., 1996). However, this explanation is flawed by laboratory experiments showing that large Daphnia are rather tolerant to high temperatures (Mitchell & Lampert, 2000). Another factor that could be involved is the quality of food for Daphnia, which may decrease towards lower latitudes as the relative share of cyanobacteria may increase at higher temperatures (e.g., Weyhenmeyer, 2001). Also, this seems unlikely to be the main explanation of the latitudinal gradient in top-down control as many low-latitude lakes are not dominated by cyanobacteria, but still lack large Daphnia. Probably the most important current hypothesis for explaining the lack of large Daphnia in tropical lakes is that the systematic shifts in the fish community with latitude are the key. In temperate regions most fish reproduce only once a year, leaving a period in spring in which there are few small (and hence planktivorous) fish, allowing large zooplankton to become abundant and filter the water clear of phytoplankton (Sommer et al., 1986). By contrast, many fish species at low latitudes reproduce more-or-less continuously (Fernando, 1994; Lazzaro, 1997; Pinel-Alloul et al., 1998) resulting probably in a continuously high predation pressure by young fish on zooplankton. There are many other aspects in which the ecology of the small omnivorous fish that dominate low latitude lakes differs from that of the relatively large cyprinids and other species that typically dominate eutrophic temperate lakes. This casts doubt on whether the large effects of nutrients and aquatic vegetation on the trophic cascade observed in temperate lakes will occur in low latitude lakes in a similar way.

Scientific significance of filling the knowledge gap

Despite an increasing interest in the functioning of low latitude lakes, (sub)tropical lakes are still poorly understood. Indeed, all of the hypothesised explanations of the lack of top-down control of algae in warm regions we discussed are based on local observations or reviews of existing literature. The systematic study of the trophic cascade in lakes along a large latitudinal gradient we propose is a novel and potentially powerful way to address these knowledge gaps. Resolving these questions is important, not only because water quality problems are severe in many of the systems we will study in South America, but also because the study of these lakes may help making a better prognosis of the potential effects of climatic change on temperate lakes.

Brooks, J.L. and Dodson, S.I. 1965. Predation, body size, and composition of plankton. Science. 150: 28-35.

Crisman, T.L., and J.R. Beaver. 1990. Applicability of planktonic biomanipulation for managing eutrophication in the subtropics. Hydrobiologia 200/201: 177-185.

Dumont, H.J. 1994. On the diversity of the Cladocera in the tropics. Hydrobiologia 272: 27-38 Fernando, C.H. 1994. Zooplankton, fish and fisheries in tropical freshwaters. Hydrobiologia 272: 105-123.

Gillooly, J.F., and S.I. Dodson. 2000. Latitudinal patterns in the size distribution and seasonal dynamics of new world, freshwater cladocerans. Limnology and Oceanography 45: 22-30.

Jeppesen, E., M. Søndergaard, J. P. Jensen, E. Mortensen, and O. Sortkjaer. 1996. Fish-induced changes in zooplankton grazing on phytoplankton and bacterioplankton: a long-term study in shallow hypertrophic Lake Søbygaard. Journal of Plankton Research 18:1605-1625.

Lammens, E.H.R.R., Gulati, R.D., Meijer, M-L. and Van Donk, E. 1990. The first biomanipulation conference: a synthesis. Hydrobiologia 200-201: 619-628.

Lazzaro, X. 1997. Do the trophic cascade hypothesis and classical biomanipulation approaches apply to tropical lakes and reservoirs? Verh.Int.Ver.theor.angew.Limnol. 26: 719-730.

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Moore, M.V., C.L. Folt, and R.S. Stemberger. 1996. Consequences of elevated temperatures for zooplankton assemblages in temperate lakes. Archiv für Hydrobiologie 135: 289-319.

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Scheffer, M. 1998. Ecology of Shallow Lakes. Chapman and Hall. London

Sommer, U., Gliwicz, Z.M., Lampert, W., & Duncan, A. 1986. The PEG-model of seasonal succession of planktonic events in freshwaters. Archiv für Hydrobiologie 106: 433-471.

Timms, R. M., and B. Moss. 1984. Prevention of growth of potentially dense phytoplankton populations by zooplankton grazing in the presence of zooplanktivorous fish in a shallow wetland ecosystem. Limnology and Oceanography 29:472-486.

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Aim:

Our aim is to find out how climate and nutrient load interact to affect the functioning of the trophic cascade at low latitudes.

Hypothesis and research questions

Our central hypothesis is that an increase in average temperature and nutrient level produce systematic changes in fish community leading to a reduction in the average size of zooplankton and a weakening of top-down control of phytoplankton, which can be ameliorated by the presence of aquatic vegetation.

This central hypothesis translates into five specific research questions:
1. Does the average body size of zooplankton (and in particular the cladocerans) diminish along the climatic gradient from temperate to tropical systems?
2. Does variation in the abundance of planktivorous fish along a latitudinal gradient explain much of the variation in zooplankton size structure?
3. Does the effect of nutrient level on apparent fish predation pressure on zooplankton change along the climatic gradient?
4. Does the effect of vegetation cover on apparent fish predation on zooplankton change along the climatic gradient?
5. Do fish kills induced by extreme meteorological events such as droughts have cascading effects on zooplankton size and phytoplankton biomass in low latitude lakes, as they do in temperate lakes?

Research:

Research methodology

We will address our hypothesis by analysing indicators of the state of the trophic cascade in a set of 100 lakes with different nutrient levels over a latitudinal gradient from Brazil to Argentina. In addition, we will use sediment cores from a subset of 20 lakes to reconstruct the history of variations in the state of the cascade in relation to changes in environmental conditions.

Our approach may seem rather ambitious, but may in fact be realized as we will combine with an (already funded) parallel WOTRO project (W 84-549) starting in 2004 with a sampling program of the same lakes. We have already started preparations and enlarged the research team by setting up a network of five research groups along the latitudinal gradient of lakes who are eager to cooperate. We have had a first meeting at which we agreed on a field protocol and a relatively extensive sampling schedule.

We will do the census in two periods. First we will sample Rio Grande southwards (until Mar del Plata) starting November 2004. Then we will sample from Rio de Janeiro northwards starting in July 2005. Finally we will sample those lakes located at the southest part of Argentina (Tierra del Fuego) in January 2006. We will have one protocol, and except for the car and boat, we will use one set of equipment for all sampling.

We will collect information along two lines:
1. A one-time mid-summer census of the state and basic properties of a selected set of about 100 lakes.
2. Sediment cores of a representative subset of 20 of the censused lakes.

The research questions will be addressed using the two sets of data (recent from census and palaeontological from cores) in a complementary way Changes in size structure of zooplankton with climate (question 1) may be verified in a straightforward way from the set of recent data, but should also be reflected in the subfossil zooplankton communities found in the cores. Our (recent) fish data will be used in conjunction with the indicators of zooplankton size structure to address research question 2. The hypothesized effect of nutrient level on the trophic cascade (question 3) will be analysed from the recent data by checking how fish abundance and zooplankton size structure changes with total-P, and other indicators of nutrient level (including sediment) in our temperate versus low latitude lakes. The palaeontological data offers an interesting other cut at the same question, as they will allow us to reconstruct how apparent predation pressure (derived from zooplankton community structure) shifts during the eutrophication process at different latitudes. Similarly, we will be able to use both recent and palaeontological data to check if the effect of aquatic vegetation on the trophic cascades is reduced at lower latitudes (question 4), as sharp shifts between vegetation dominated and vegetationless states which tend to occur in such lakes can be reconstructed well from sediment cores. Finally, the palaeontological data will allow us to scan for cascading effects of possible fish kills induced by extreme meteorological events at different latitudes (question 5). Such kills due to desiccation of lakes or anoxia can be considered natural biomanipulation experiments, and may give us a clue to differentiate between (sub)tropical and temperate lakes responses to biomanipulation as a management measure.

Funding via NWO

Yes

External Funding Sponsors

WOTRO (NWO)

Research Group

WUR - Aquatic Ecology and Water Quality Management